2,6- or 2,7-naphthalenedicarboxylic acid obtained by oxidation of dimethylnaphthalenes, particularly 2,6- or 2,7-dimethylnaphthalene, is used as a starting material for production of polyesters such as polyethylene naphthalates. These polyesters provide synthetic fibers and films having excellent characteristics. It has therefore been desired to develop a process for producing 2,6- or 2,7-dimethylnaphthalene at low costs and with high quality.
Dimethylnaphthalenes are contained in coal tar or a cycle oil in the fluid catalytic cracking process and, thus, a method of recovering dimethylnaphthalenes by distillation of coal tar or the cycle oil has been proposed (see, for example, JP-A-60-69042 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")).
In the method of recovering dimethylnaphthalenes from coal tar or the cycle oil in the fluid catalytic cracking process, since the cycle oil contains a high concentration of nitrogen and sulfur compounds, the dimethylnaphthalenes obtained are contaminated with these nitrogen and sulfur compounds. These nitrogen and sulfur compounds accelerate catalyst poisoning in isomerization of substituted dimethylnaphthalenes other than 2,6- or 2,7-dimethylnaphthalene, and further in adsorption separation of 2,6- or 2,7-dimethylnaphthalene by the use of a zeolite and so on. It is therefore necessary to decrease the amounts of nitrogen and sulfur compounds in dimethylnaphthalenes to about 10 ppm or less. Concerning hydrotreating to decrease the amounts of the nitrogen and sulfur compounds to about 10 ppm or less, it should be carried out under severe conditions. Hydrotreating under such severe conditions inevitably causes hydrogenation and cracking of dimethylnaphthalenes, resulting in a great reduction in yield of dimethylnaphthalenes. Thus, additional dehydrogenation is needed, and a problem arises in that the production cost is markedly increased.
Normal paraffins are recovered from a kerosene fraction as a starting material for production of linear alkylbenzene sulfonates (LAS) as synthetic detergents.
As a result of investigations, it has been found that a product oil obtained by reforming reaction of the above-described raffinate contains a large amount of dimethylnaphthalenes and that the product is almost free from nitrogen and sulfur compounds.
It is reported that catalytic reforming of a kerosene fraction provides heavy aromatic compounds and that the heavy aromatic compounds conta in dimethylnaphthalenes (Sekiyu Gakkaishi, Vol. 13, No. 6 (1970), pp. 468-474). But, astonishingly, by the reforming reaction of the raffinate, dimethylnaphthalenes can be formed in an amount of about 1.5 times that in the reforming reaction of the kerosene fraction.